Past studies on the link between smoking cannabis and lung cancer have produced mixed results, says Beasley, and a key problem has been separating out the contribution of tobacco, which is often mixed with cannabis.

He says the practice of mixing tobacco with cannabis does not tend to occur in New Zealand, unlike in other places such as the UK, and the study controlled for tobacco use in all forms.

Beasley and team interviewed 79 lung cancer patients and sought to identify the main risk factors for the disease, such as smoking, family history and occupation. They then asked the patients about alcohol and cannabis consumption.

The researchers found those who smoked more than a joint a day for 10 years, or two joints a day for 5 years were more than 5 times more likely to have lung cancer than those who didn't smoke at all - after adjusting for other variables.